This invention relates to methods of dry-forming fibrous products. More particularly, the invention is directed to methods for bonding loose fibrous webs and for bonding fibrous webs which have some integrity.
It has heretofore been a practice in dry-forming systems to dry-form a loose continuum of fibers, hereafter referred to as a loose fibrous web or loose web on a moving foraminous forming surface, and because the loose fibrous web is very weak, to bond its fibers and give it integrity sufficient for its handling and processing into a desired fibrous web product. One method of bonding the fibers is to include a mix particulate adhesive material with the fibers, deposit them together onto the forming surface and then activate the adhesive with water and/or heat. Another method is to spray adhesive onto the fibers as they are airborne within a distributor or as they are in transit from a distributor to the forming surface. Still another technique is to dry-lay the fibers onto a moving foraminous carrier wire or screen and, at a first bonding station (hereafter termed Bond I), spray a binder solids-bearing emulsion onto one side of the dry-laid loose fibrous web; heat the adhesive solids-bearing loose web (at a station termed Dry I) to at least partly dry and perhaps partly cure the binder and form a once-bonded fibrous web having some integrity; transfer the once-bonded web to a second moving foraminous carrier wire while inverting the web, and, at a second bonding station (hereafter termed Bond II), spray the emulsion onto the other side of the web. The twice-bonded web is again heated to partly or fully dry the binder and then the binder is cured to provide a fully bonded fibrous web product. This latter technique is described in a portion of commonly assigned pending Patent Application Ser. No. 081,791. In accordance with the latter technique, the emulsion, usually a water-based latex material, is applied in the same amounts and at the same adhesive solids concentrations to each side of the web. For example, if it were desired to manufacture a fibrous web product suitable for use as a wiper and having a basis weight of 45 lbs. per 3000 sq. ft. ream, of which 10 lbs. were binder solids, roughly the same amount of same the emulsion of a certain solids concentration would be sprayed onto each side of the web to add 5 lbs. of solids to one side and 5 lbs. of solids to the other side of the web.
While the aforementioned technique is commercially satisfactory, it is an object of this invention to improve certain aspects of the technique. With respect to dry-forming systems which employ two foraminous carriers, one to carry the loose web through Bond I, and one to carry the adhesive-bearing loose web through Dry I, this invention seeks to reduce the water content of the emulsion sprayed onto the loose web at Bond I because as the water content of the loose web increases, problems in transferring the web from one carrier to the other also increase. The wetter the web, the more it tends to stay on a carrier. In such interrupted carrier systems and those systems wherein a common carrier is employed between Bond I and Dry I, the water content of the web increases the load imposed on the dryer at Dry I. With a common carrier, the binder solids added on by the emulsion at Bond I require a certain volume of water to wash the carrier free of solids before a portion of the carrier can be reused to carry more freshly deposited loose fibers. Some of this wash water is carried with the carrier into the dryer and increases its load. Thus, for common or double carrier systems, this invention aims to reduce the add-on level of the emulsion sprayed on at Bond I and the volume of wash water needed at the Bond I, Dry I stations. This would reduce the load imposed on the dryer at Dry I.
This invention also endeavors to reduce the total binder solids used to bond air laid webs, and to reduce solids content of the emulsion used at Bond I to the extent that still permits the formation of once-bonded webs which are commercially transferable from station to station. Having discovered that the efficiency of latex solids as a binder for air laid webs increases if and to the extent that a vacuum is drawn from under the carrier, preferred embodiments of this invention uniquely utilize a vacuum, as high a vacuum as possible, near Bond II to increase solids penetration into the web and thereby increase latex solids efficiency.
As compared to the previously-practiced double spray application bonding technique, the methods of this invention also involve applying the binder solids bearing emulsion at the same or different binder solids concentrations and at different solids add-on levels at each Bond station. In accordance with preferred methods of this invention, at Bond I, a minor add-on of emulsion having a relatively high 20% to 30% binder solids concentration based on the weight of the emulsion, applies only about 20 to about 40% of the total adhesive solids to be applied to the web.
At Bond II, a major add-on of emulsion having about a 15% by weight binder solids concentration applies the rest, i.e. about 60% to about 80% of the total binder solids to be applied to the web. Thus, for example, to manufacture a 45 lbs. basis weight wiper whose weight includes a total of about 10 lbs. of binder solids, at Bond I, a low add-on of emulsion having about a 20% solids concentration would apply about 3.3 lbs. of binder solids, and at Bond II, a greater add-on of emulsion having about 15% solids concentration would apply about 6.6 lbs. of binder solids to the web.
The previously referred to endeavors of improvement are met because providing a low emulsion add-on level at Bond I, minimizes the web water content there and consequently alleviates web transfer problems in double carrier systems and minimizes dryer loads for both common and double carrier systems. Reducing the binder solids concentration from the conventional level of 50% to from about 20% to about 40% reduces the wash water requirements which also reduces dryer loads. Providing a higher solids add-on level at Bond II where a high vacuum can be drawn, maximizes binder solids efficiency and thereby permits the production of webs and products having less total binder solids than previously. This reduces binder solids containing bonding material and curing energy costs. Even when the total binder solids add-on is the same for wiper products made according to the methods of this invention as for wiper products produced by the previous double adhesive application techniques, it has been found that tensile strengths of the former are greater than those of the latter; it has also been found that drawing a vacuum in cooperation with the second application obtains web products whose tensile strength values are greater than if no vacuum were so drawn. Further, wiper products made in accordance with the methods of the invention have an improved feel and a different appearance than those made by the previous 50%--50% add-on techniques. The wiper's Bond I low solids add-on side feels smoother and softer than the Bond II high solids add-on side, or than the Bond I side of the previous wiper. When a high vacuum is drawn at Bond II, the Bond II high solids add-on side tends more to take on the pattern of the foraminous carrier and have more of a cloth-like appearance than its Bond I side, or that side of the previous wiper.
It is an object of this invention to provide methods for bonding loose fibrous continuums or webs, especially dry-formed ones.
Another object of this invention is to provide methods of applying binder solids-bearing bonding materials onto loose fibrous continuums or webs, especially dry-formed ones.
Another object is to provide improved methods of applying binder solids bearing emulsions onto both sides of dry-formed fibrous webs.
Another object is to provide the aforementioned methods which reduce the total quantity of solids needed to bond such webs.
Another object is to provide the aforementioned methods which utilize a vacuum to increase bonding material penetration, efficiency and web product tensile strength.
Still another object of this invention is to provide the aforementioned methods which reduce the cost of producing such webs.